Polyethylene may be classified into two broad families, namely “random” (which is commercially prepared by initiation with free radicals under polymerization conditions that are characterized by the use of very high ethylene pressures) and “linear” (which is commercially prepared with a transition metal catalyst, such as a “Ziegler Natta” catalyst, or a “chromium” catalyst, or a single site catalyst or a “metallocene catalyst”).
Most random polyethylene which is commercially sold is a homopolymer of ethylene. This type of polyethylene is also known as “high pressure low density polyethylene” because the random polymer structure produces a lower polymer density. In contrast, most linear polyethylene which is commercially sold is copolymer of ethylene with at least one alpha olefin (such as, butene, hexene or octene). The incorporation of a comonomer into linear polyethylene reduces the density of the resulting copolymer. For example, a linear ethylene homopolymer generally has a very high density (typically greater than 0.955 grams per cubic centimeter (g/cc))—but the incorporation of small amounts of comonomer results in the production of so-called “high density polyethylene” (or “HDPE”—typically, having densities greater than 0.940 g/cc) and the incorporation of further comonomer produces so-called “linear low density polyethylene” (or “LLDPE”—typically having a density of from about 0.905 g/cc to 0.940 g/cc).
Linear polyethylene is converted into finished goods using a variety of molding and extrusion processes. Additives are typically used to improve the conversion process and/or to modify the properties of the finished good.
One widely used family of additives may be described as the mono- and di-esters of glycerol with higher fatty acids. Examples of these additives include glycerol monostearate (also known as glyceryl monostearate and/or “GMS”) and glycerol monooleate.
GMS may be used as a (i) mold release agent (including rotational molding); (ii) an anti-static agent (especially for film); or (iii) as a blowing co-agent for the preparation of foams.
GMS is very polar in comparison to the host polyethylene resin. Accordingly, GMS tends to migrate from the resin and form a coating on the surface of the finished polyethylene goods. This is desirable in some respects (as this coating is believed to be responsible for the mold release and anti-static properties) but it is also undesirable because the GMS can also leave a greasy residue on surfaces that come into contact with the molded polyethylene part or film (such as, the mold shell and/or film extrusion equipment).
A need exists for materials and methods to reduce the amount of greasy residue that is transferred to polyethylene conversion equipment when GMS is used as an additive.